The present invention relates to methods for attaching integrated circuit dies to substrates.
Silicon dies having integrated circuits created therein are typically attached to a heat spreader or substrate. Wire bond is electrically fixed to the silicon die and bonding pad on the substrate to communicate electrical signals from the integrated circuits within the die to electronic circuits external to the die. The attachment of the silicon die to the substrate or heat spreader is of critical importance. The methods and materials used to attach the die to the substrate have a significant impact on the performance and reliability of the silicon die.
One prior art method for attaching the silicon die to the substrate utilizes high temperature solder material. While this method accomplishes its intended purpose, significant problems still exist. For example, attaching the silicon die with high temperature solder induces stresses in the die upon thermal cycling. These stresses are caused by the different coefficients of thermal expansion between the silicon die, solder, and substrate. Consequently, the induced stresses lead to early failure of the die.
Other methods for attaching the silicon die to the substrate to overcome the disadvantages stated above have been developed. For example, an improved method for attaching the silicon die utilizes low temperature solder. While this method for attaching the silicon die to the substrate or heat spreader eliminates the stresses induced in the die and thus significantly increases the die""s life, significant problems still exist. For example, the use of this method creates voids and cracks in the solder material. The voids and cracks significantly increases thermal resistance in the solder material. Consequently, the increased thermal resistance raises the junction temperature and ultimately leads to premature failure of the silicon die.
Therefore, there is a need for a new and improved method for attaching a silicon die to a substrate. This new and improved method must reduce or eliminate thermal stresses in the silicon die and must increase the life in service of the silicon die.
In accordance with an aspect of the present invention, a new and improved method for attaching silicon die to a substrate is provided. The method of the present invention creates a void-free attachment of the silicon die to the substrate. More specifically, a low temperature die attachment material (i.e., solder) is used to affix the silicon die to the substrate.
In an embodiment of the present invention, the void-free attachment of the silicon die to the substrate is accomplished by exerting a downward force on the silicon die directed toward the substrate to ensure good contact between the die and the substrate through the low temperature die attachment material.
In another embodiment of the present invention, the force is created by clamping the die to the substrate using a pair of mechanical clamps.
In another embodiment of the present invention, a spring is disposed between a silicon die cover and the die. The spring exerts a downward force on the die directed toward the substrate, thus preventing air from becoming trapped in the solder.
Further objects, features and advantages of the invention will become apparent from consideration of the following description and the appended claims when taken in connection with the accompanying drawings.